Photovoltaic (PV) cells used for photoelectric conversion, e.g., silicon solar cells, are relatively small in size, typically measuring 2–4″ on a side in the case of cells made from rectangular EFG-grown substrates. It is common industry practice to combine a plurality of PV cells into a physically integrated PV module having a correspondingly greater power output. Also, several PV modules may be connected together to form a large array with a correspondingly greater power output. Typically PV modules are formed from 2 or more “strings” of solar cells, with each string consisting of a plurality of cells arranged in a row and electrically connected in series, and the several strings being arranged physically in parallel with one another so as to form an array of cells arranged in parallel rows and columns with spaces between adjacent cells. The several strings are electrically connected to one another in a selected parallel or series electrical circuit arrangement, according to voltage and current requirements.
For various reasons, it has been common practice for the modules to be laminated structures. These laminated structures consist of front and back protective cover sheets, with the front sheet being stiff and made of clear glass or a suitable plastic material that is transparent to solar radiation, and the back cover sheet (commonly called a “backskin”) being made of the same or a different material as the front sheet. Disposed between the front and back sheets in a sandwich arrangement are the interconnected solar cells and a light-transparent polymer material that encapsulates the solar cells and also is bonded to the front and back sheets so as to physically seal off the cells. The laminated structure provides mechanical support for the cells and also protects them against damage due to environmental factors such as wind, snow, and ice. Another common practice is to fit the laminated module into a metal frame, with a sealant covering the edges of the module engaged by the metal frame. The metal frame protects the edges of the module, provides additional mechanical strength, and facilitates combining it with other modules so as to form a larger array or solar panel that can be mounted to a suitable support that holds the modules at the proper angle to maximize reception of solar radiation.
When a plurality of cells are arrayed in a module, the total active surface area of the array (i.e., the active area of the front faces of the solar cells) is less than the total area exposed to radiation via the transparent front protective sheet. For the most part this is due to the fact that adjacent cells do not touch each other and also the cells at the periphery of the array may not extend fully to the outer edges of the front protective sheet. Consequently, less than all of the solar radiation that is received by the PV module impinges on active solar cell areas, with the remainder of the received solar radiation impinging on any inactive areas that lie between the cells or border the entire array of cells.
A number of techniques have been proposed for increasing the efficiency and effectiveness of PV modules by concentrating incident solar radiation onto active cell areas. U.S. Pat. No. 4,235,643, issued Nov. 25, 1980 to James A. Amick for “Solar Cell Module”, discloses a solar cell module characterized by a solar cell support having a plurality of wells for receiving individual solar cells, with the surface of the support between the cells having a plurality of light-reflective facets in the form of V-shaped grooves, with the angle at the vertex formed by two mutually-converging facets being between 110° and 130°, preferably about 120°, whereby light impinging on those facets would be reflected back into the transparent cover layer at an angle Ø greater than the critical angle, and then reflected again internally from the front surface of the cover layer so as to impinge on the solar cells. The term “critical angle” refers to the largest value that the angle of incidence may have for a ray of light passing from a denser optical medium to a less dense optical medium. As is well known, if the angle of incidence Ø exceeds the critical angle, the ray of light will not enter the less dense medium (e.g., air) but will be totally internally reflected back into the dense medium (e.g., the transparent cover layer 54 of Amick).
U.S. Pat. No. 5,994,641, issued Nov. 30, 1999 to Michael J. Kardauskas for “Solar Module Having Reflector Between Cells”, discloses an improvement over the Amick invention by incorporating a light-reflecting means in the form of an optically-reflective textured sheet material in a laminated module comprising a transparent front cover, a back cover, a plurality of mutually spaced and electrically-interconnected photovoltaic cells disposed between the front and back covers, and a transparent encapsulant material surrounding the cells and bonded to the front and back covers. The optically-reflective sheet material is disposed between the cells and also between the cells and the outer periphery of the module. The optically-reflective textured sheet material of Kardauskas comprises a substrate in the form of a thin and flexible thermoplastic film and a light reflecting coating on one side of the substrate, with the substrate being textured by embossing so as to have a plurality of contiguous v-shaped grooves characterized by flat mutually-converging surfaces (“facets”) that extend at an angle to one another in the range of 110°-130°, preferably about 120°. The light-reflecting facets extend in a predetermined angular relationship with respect to the front cover, so that light impinging on that those facets will be reflected upwardly back through the covering transparent encapsulant and the glass to the glass interface with air, and then backwards through the glass and covering encapsulant toward active areas of the cells. Kardauskas teaches that the light-reflecting coating may be either a light-reflecting metal film or a dielectric stack comprising multiple layers of materials arranged to form a reflecting mirror.
The Kardauskas patent teaches the use of a reflective material having a linear pattern of grooves wherein all of the grooves are parallel to one another. It also teaches that the embossed linear pattern of grooves may be replaced by an embossed herringbone pattern of grooves. In one arrangement, a sheet of the reflective material with a linear pattern of grooves is placed between the cells and the backskin, with the sheet being large enough so that it extends beyond the perimeter of the array of cells. The grooves of that sheet extend in the same direction as the columns or rows. Then additional strips of the same material are placed over the larger sheet in those portions of the land areas between the cells so that the grooves form a pattern wherein certain of the grooves extend parallel to the cell rows and other grooves extend parallel to the cell columns. More precisely, the linear grooves between adjacent rows are oriented at a right angle to the grooves between columns, so as to improve the amount of light that is internally reflected from the areas between the cells back onto the front surfaces of the cells. Kardauskas teaches a second way to obtain a patterned groove arrangement using his reflective material with linear pattern of grooves. The second way comprises cutting the reflective sheet material with a linear groove pattern into a plurality of strips, with individual strips being placed between adjacent columns and other strips being placed between adjacent rows so that the grooves of the strips between rows extend at a right angle to the grooves of the strips between columns.
The advantage of the Kardauskas invention is that it provides a material improvement in power output. As disclosed in the Kardauskas patent, a plurality of test cell coupons, each comprising a square cell measuring 100 mm on each side and surrounded on each side by a 25 mm. wide strip of laminated reflective film material, with the 0.002″ deep V-shaped grooves of that material running in one direction along two opposites sides of the cell and running in a second direction at a right angle to the first direction along the other two sides of the cell, were found to show improvements in power output in the range of 20.8% to 25.6% when illuminated by a solar simulator light source. A limitation of the Kardauskas invention is that introduces an additional separate component to the module and the laminating process.
Japan Published Patent Application No. 62-101247 discloses the concept of providing a solar cell module with a reflective back cover sheet in the form of a laminate comprising a polyester base layer and a light-reflecting aluminum coating, with the back cover sheet having a plurality of V-shaped grooves that provide angular light reflecting facets. The cells are spaced from one another in front of the back sheet, so that incident light passing through the front cover sheet and between the cells is reflected by the back cover sheet back to the transparent front cover sheet.
The module is made by placing the front and back cover sheets, the cells and an encapsulant in the form of EVA (ethylene-vinyl acetate polymer) in a laminating apparatus having an embossing platen, with the back cover sheet comprising a flat polyester base layer and an aluminum coating on the front side of the base layer. The polyester base layer faces the embossing platen. When the apparatus is operated to form a laminated solar module as described, it subjects the assembled components to heat and pressure, causing the encapsulant to melt and the platen to emboss a linear grooved pattern into the back cover sheet, with the result that back cover sheet has grooves on both its front and back sides. The manufacturing procedure taught by Japan Published Patent Application No. 62-101247 has two limitations.
First of all, the aluminum cover sheet does not directly engage the embossing platen, and hence the precision with which grooves are embossed in the aluminum coating depends on how precisely the platen can emboss grooves in the polyester base layer. Secondly, polyester films have a high melt temperature, typically about 250 degrees C. or higher, and must be heated under pressure to at least their melt point in order to be permanently deformed into a pattern of grooves by the embossing platen. Conventional polyester, i.e., polyethylene terephthalate (PET) has a melt point of about 250 degrees C., while poly 1,4 cyclo-hexanedimethanol terephthalate (PCT) has a melt point of 290 degrees C. However, EVA encapsulant has a melt point of about 150 degrees C. and will deteriorate if over heated. Consequently the laminating and embossing temperature must be limited to avoid overheating the EVA, but limiting that temperature has the effect of making it more difficult to permanently and precisely emboss the polyester base with sharp V-shaped grooves that are replicated in the aluminum coating. If the grooves formed in the aluminum coating do not have flat sides that converge at the required angle, less light will be internally reflected in the desired mode, thereby limiting any increase in module power output resulting from the use of a reflective back sheet.
The design disclosed by Japan Published Patent Application No. 62-101247 has been utilized in PV modules made by Sharp Corp. and described in an article published by the CADDET Japanese National Team entitled “Bigger Gaps Make PV Cells More Efficient”. That publication indicates includes a drawing illustrating a module with a grooved reflective back cover sheet as disclosed by Japan Published Patent Application No. 62-101247. However, that article also indicates that an improvement of cell power output of only 4% is achieved by Sharp modules having the multi-groove reflecting back cover sheet. One possible explanation for the failure to achieve a greater improvement in power output may be how the reflective back sheet is made. Another explanation is that the grooves all extend in the same direction.